What about having the unobtanium (like the name btw) be specifically useful to pvp?
Say smelting ore. You can use the ore to make iron arrow heads that are stronger than obsidian or flint arrow heads. Having access to this ore gives a pvp tribe in the pvp area a good advantage over a wanted resource that shreds through armor more effectively.
The attacking tribes that want to take over that resource can still use obsidian arrow heads, but they will need a higher quantity since they are not as effective.
Obsidian is more plentiful in the safe area. It's adequate for hunting and such. A pvper doesn't want to waste too much time in the safe area (even with alt) having to gather obsidian so they trade with the safezone folks for it.
Say tar is only available in the pvp area. It's useful for trebuchets and other siege weapons. It can be poured over walls to stop an incursion. Tar is something tribes want to fight over for the strategic advantage.
People in the safe area still want it though, to make torches. Easier to get leather in the safe area as hunting is a bit safer so they trade the tar for the leather.
Just examples. Kind of follows the EvE model, I think.
As Jadzia mentioned elsewhere, food production might be easier in the safe area and let's face it... tending the field is a bit carebearish so that seems appropriate. Food that is very perishable becomes a great commodity for trade.
One question though (because I'm not creative enough to think of more
)
Why have the unobtanium automatically transported?
Would this not cut down on the chances for ambush and other pvp goodness?
I'm thinking there should be specific areas allowing trade between safe and unsafe so that pvpers have a chance to be vulnerable and exposed more often. It should require a great number of them to safely transport cartfulls of maize back to the castle, and an adequate number to mount a successful ambush.